In recent years, the environmental regulation on a global scale is becoming severe more and more, and the circumstances surrounding automobiles are getting severe from the aspects of fuel consumption regulations, exhaust gas regulations, and so on. In particular, an improvement of fuel consumption performance of vehicles, such as automobiles, etc., is a big issue. As one means for solving that issue, an improvement in fuel saving properties of a lubricating oil composition to be used for vehicles is demanded.
For improving the fuel saving properties of a lubricating oil composition, a polymethacrylate (PMA) is, in general, used as a viscosity index improver to be blended in the lubricating oil composition.
However, in general, it is known that when a lubricating oil composition including a viscosity index improver, such as PMA, etc., is used under high temperature high shear viscosity conditions, the detergency is worsened. Therefore, it is performed to increase a blending amount of a metallic detergent in the lubricating oil composition, or to investigate a suitable combination of metallic detergents.
For example, PTL 1 proposes a lubricating oil composition in which predetermined amounts of a nitrogen-containing ashless dispersant, a metal-containing detergent, an alkali metal borate hydrate, and specified zinc dihydrocarbyl dithiophosphate are blended together with a viscosity index improver, such as PMA, an ethylene-propylene copolymer, etc., in a lubricating base oil, and the blend is dissolved or dispersed.